Method of making Tl-Sr-Ca-Cu-oxide superconductors comprising heating at elevated pressures in a sealed container

ABSTRACT

High T c  superconducting compounds are made by forming a reaction mixture of the oxides of Sr, Cu, Ca and Tl, compressing these into a hardened body, and placing the hardened body into a container. The container is then evacuated and sealed. The hardened body is heated under pressure until the oxides of Sr, Ca, Cu, and Tl react to form a superconducting compound.

BACKGROUND OF THE INVENTION

1. Field of the invention

This invention relates to a new copper-oxide high T_(c) (above 100 K) superconductor, Tl-Sr-Ca-Cu-oxide.

2. Description of the Background Art

Since 1987, there has been considerable research on high T_(c) copper oxide superconductors. Superconductors with T_(c) 's greater than 77 K include, for example, Y(RE)Ba₂ Cu₃ O₇, Bi₂ Sr₂ CaCu₂ O_(x) and Tl₂ Ba₂ Ca₂ Cu₃ O₁₀, this latter compound having the highest known T_(c) (C. C. Torardi, M. A. Subramanian, J. C. Calabrese, J. Gopalakrishnan, K. J. Morrissey, T. R. Askew R. B. Flippen, U. Chowdhry, and A. W. Sleight. Science 240, 631-634 (1988)). While these compounds are complex layered structures, only a very limited number of combinations of elements are favorable for superconductivity. In addition to Cu and O, which all known high T_(c) compounds contain, the known favorable combinations are 1) a rare earth element (RE) and Ba, 2) Bi, Sr and Ca and 3) Tl, Ba and Ca. Numerous attempts to expand the list of combinations have failed. Although Ba and Sr are chemically similar, attempts to substitute Sr for Ba and vice-versa in the above combinations have been unsuccessful. For example, attempts to substitute Sr for Ba in Tl-Ba-Ca-Cu-O systems to fabricate superconducting Tl-Sr-Ca-Cu-O systems using current conventional ceramic processing techniques have all failed. Although Sr has been partially substituted for Ba in Tl compounds, it has resulted in a substantial lowering of T_(c) (M. A. Subramanian, C. C. Torardi, J. Gopalakrishnan, P. L. Gai, J. C. Calabrese, T. R. Askew, R. B. Flippen, and A. W. Sleight. "Bulk Superconductivity Up To 122 K In The Tl-Pb-Sr-Ca-Cu-O System"). One probable reason for this is that Ba and Sr have different ionic sizes which causes the analog compounds to be energetically unfavorable.

In addition to the inability to substitute Sr for Ba in forming superconducting compounds, there is also a problem in forming superconducting Tl containing compounds. A major problem with any Tl-based compound is its toxicity. Tl has a relatively high vapor pressure at temperatures needed to react superconducting compounds.

SUMMARY OF THE INVENTION

Accordingly, it is an object of this invention to form superconducting compounds with T_(c) 's in excess of 100 K.

It is also an object of this invention to form a Tl-Sr-Ca-Cu-oxide superconducting compound.

It is a further object of this invention to eliminate the Tl-vapor contamination problem.

These and additional objects of the invention are accomplished by forming high T_(c) superconducting compounds. A reaction mixture of the oxides of Sr, Cu, Ca and Tl is formed, compressed into a hardened body, and placed into a container. The container is then evacuated and sealed. The hardened body is heated under pressure until the oxides of Sr, Ca, Cu, and Tl react to form a superconducting compound.

DETAILED DESCRIPTION OF THE INVENTION

The starting materials of the invention are the oxides of strontium, calcium, copper and thallium, in the correct stoichiometric proportions to make the desired compound, a Tl-Sr-Ca-Cu-oxide superconducting compound having a T_(c) above 100 K. Preferably, the superconducting compound is selected from the group comprising TlSr₂ CaCu₂ O. and TlSr₂ Ca₂ Cu₃₋₄ O_(x) wherein x represents a fully oxygenated compound. Most preferably, x is between about 7 and 17.

Although the starting materials can be mixed to form a reaction mixture to make the superconducting compound, it is preferable to obtain the oxides strontium and calcium by converting the carbonates of strontium and calcium to their respective oxides. Most preferably, the powders of SrCO₃, CuO, and CaCO₃ are mixed and this powdered mixture is calcined in order to convert the carbonates to oxides. Tl₂ O₃ is then added to this mixture to form a reaction mixture.

If a powdered mixture of SrCO₃, CuO, and CaCO₃ is calcined to obtain the oxides, it is preferable to calcine the powdered mixture in an air or oxygen atmosphere at a temperature from about 800° C. to about 1000° C. for between about 3 to about 10 hours, long enough to convert the carbonates to oxides. Actually, the time required to calcine the mixture depends on the particle size of the powder, since smaller particles will require less time than larger particles. Most preferably, the powder mixture is subjected to a temperature of from about 910° C. to about 940° C. for about 3 hours in an air atmosphere.

The reaction mixture should be finely ground. This is because an increased surface area allows for greater efficiency of reaction, decreasing reaction time and increasing product yield. Preferably, the reaction mixture consists of particles having an average diameter of from about 2 to about 10 microns in diameter.

The reaction mixture is then compacted into some type of hard body. This is done so that the reaction mixture can be easily worked and also because in subsequent reaction steps, loose powders have a tendency to either migrate into their containers or weld to their containers. Although the reaction mixture can be compacted into any form, depending on the ultimate application for the superconducting compound, it is preferable to compact the reaction mixture into a pellet. Most preferably, the pellet is in the shape of a rod about 13 mm long and about 6 mm in diameter.

The hard body is then sealed in a container that will effectively seal the pellet from the outside environment and will prevent any of the reactants, such as toxic thallium vapors, from escaping. Preferably, the hard body is placed in a non-reactive container that, when evacuated, sealed and heated, will melt around the hard body. Examples of such containers are pyrex tubes and stainless steel envelopes. In the latter embodiment, it is preferred to wrapped the hard body in platinum foil to minimize any unwanted reactions between the pellet and the stainless steel. Most preferably, a glass tube made of Pyrex® (Corning Glass Works registered trademark for borosilicate glass) is used as a container.

The container is evacuated to as low a pressure as possible so that, when the container melts, it will effectively seal the hard body from the surroundings. Preferably, a pressure of at least about 20 mtorr is sufficient. Then the container is sealed.

The hard body, now in the sealed container, is then heated under pressure until the oxides of Tl, Sr, Ca, and Cu react to form the superconducting compound. Preferably, a temperature from about 800° C. to about 950° C. at a pressure of at least about 30,000 psi is sufficient for reaction to occur. However, reaction conditions will vary within this range depending on the container used. Some containers may melt around the pellet quickly enough at the reaction temperature so that no pre-heating of the container is required. For instance, if a stainless steel envelope is used for the container, preferably the hard body in the container is heated at a temperature from about 800° C. to about 950° C. and a pressure of at least about 30,000 psi for from about 50 minutes to about 80 minutes. On the other hand, some containers may require preheating, in order to melt the container around the hard body. For example, if the hard body is sealed in pyrex, it is preferable to first pre-heat the hard body in the container to a temperature from about 700° C. to about 800° C. at a pressure of at least about 100 psi for from about 20 minutes to about 40 minutes. This is because, unlike stainless steel, Pyrex® takes longer to melt and surround the pellet. After the Pyrex® melts, the heat is increased to from about 850° C. to about 950° C. while simultaneously increasing the pressure to at least about 30,000 psi over a period of from about 25 minutes to about 40 minutes.

Any reaction apparatus that will provide the necessary heat and constant pressure can be utilized for the above process. Preferably, a hot isostatic press (HIP) is used.

Following reaction, the superconducting compound is cooled to about room temperature and returned to atmospheric pressure over a period of from about 15 minutes to about 30 minutes. The container is removed from the superconducting compound.

Having described the invention, the following examples are given to illustrate specific applications of the invention. These specific examples are not intended to limit the scope of the invention described in this application.

EXAMPLE I

Amounts of SrCO₃, CuO, CaCO₃, and Tl₂ O₃ were weighted out in proportions to form an approximate stoichiometry of TlSr₂ CaCu₂₋₃ O_(x), where x would be the amount of oxygen taken up by the sample during the processing. Samples were prepared by finely grinding and mixing the SrCO₃, CuO and CaCO₃ and calcining the sample mixture at 915 C for 3.5 hours. The resulting compound was reground and mixed with the Tl₂ O₃.

This mixture was then pressed into a rod 13 mm long and 6 mm in diameter, wrapped in a platinum foil and sealed in an evacuated (20 mtorr) stainless steel envelope. The sample was reacted at 850 C and 30,000 psi for 1 hour in a hot isostatic press (HIP) and then returned to room temperature and atmospheric pressure in 15 minutes The stainless steel envelope was removed from the resulting product.

X-ray fluorescent, SEM/EDX, and electron microprobe analysis indicated a composition of TlSr₂ CaCu₂ O_(x). Indium leads were soldered to the sample and four-probe ac (25.5 Hz) resistance measurements were made as a function of temperature. The material exhibited metallic behavior from room temperature to 109 K at which point the onset of superconductivity occurred. The superconductive transition was reasonably sharp: by 106 K, the resistance had decreased by 90 % and by 104 K, it was immeasurably small.

Temperature dependent dc susceptibility measurements were performed in a magnetic field of 200 G. The material appeared to be slightly paramagnetic immediately above the superconductive transition and showed two distinct diamagnetic transitions occurring at 119 K and 70 K. The signal saturated below 50 K at a value which corresponded to expulsion of the magnetic field from 20 % of the sample volume.

EXAMPLE II

Amounts of SrCO₃, CuO, CaCO₃, and Tl₂ O₃ were weighed out in proportions to form an approximate stoichiometry of TlSr₂ CaCu₂₋₃ O_(x), where x would be the amount of oxygen taken up by the sample during the processing. Samples were prepared by finely grinding and mixing the SrCO₃, CuO and CaCO₃ and calcining the sample mixture at 915 C for 3.5 hours. The resulting compound was reground and mixed with the Tl₂ O₃.

This mixture was then pressed into a rod 13 mm long and 6 mm in diameter and sealed in an evacuated (20 mtorr) Pyrex® container. The sample was heated at 750 C at 100 psi in a hot isostatic press (HIP) for 30 minutes. The temperature and pressure were then increased to 850 C and 30,000 psi for 30 minutes in the HIP to allow the mixture to react. The resulting product was returned to room temperature and atmospheric pressure in 15 minutes. The Pyrex® container was removed from the resulting product.

X-ray fluorescent, SEM/EDX, and electron microprobe analysis indicated a composition of TlSr₂ CaCu₂ O_(x). Indium leads were soldered to the sample and four-probe ac (25.5 Hz) resistance measurements were made as a function of temperature. The material exhibited metallic behavior from room temperature to 109 K at which point the onset of superconductivity occurred. The superconductive transition was reasonably sharp: by 106 K, the resistance had decreased by 90 % and by 104 K, it was immeasurably small.

Temperature dependent dc susceptibility measurements were performed in a magnetic field of 200 G. The material appeared to be slightly paramagnetic immediately above the superconductive transition and showed two distinct diamagnetic transitions occurring at 119 K and 70 K. The signal saturated below 50 K at a value which corresponded to expulsion of the magnetic field from 20 % of the sample volume.

Obviously, many modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described. 

We claim:
 1. A method of forming a Tl-Sr-Ca-Cu-oxide high T_(c) superconductor comprising the steps of:forming a reaction mixture of the oxides of Sr, Cu, Ca, and Tl in stoichiometric proportions to make a Tl-Sr-Ca-Cu-oxide high T_(c) superconducting compound; compressing the reaction mixture into a hard body; placing the hard body into a container for containing thallium vapor; evacuating and sealing the hard body in the container; heating the hard body and the container at a temperature of about 800° C. to about 950° C. and under pressure of at least about 30,000 psi until said container metal around said hard body and the oxides of Tl, Sr, Ca, and Cu react to form a superconducting compound; and cooling the superconducting compound to room temperature and returning the superconducting compound to atmospheric pressure.
 2. A method as described in claim 1 wherein the Tl-Sr-Ca-Cu-oxide high T_(c) superconducting compound is selected from the group comprising TlSr₂ CaCu₂ O_(x) and TlSr₂ Ca₂ Cu₃₋₄ O_(x), wherein x is an integer from about 7 to about
 17. 3. A method as described in claim 1 wherein the oxides of Tl, Sr, Ca, and Cu are reacted by heating the hard body and the container at a temperature from about 800° C. to about 950° C. at a pressure of at least 30,000 psi.
 4. A method as described in claim 3 wherein the container is made of a material selected from the group consisting of stainless steel and a borosilicate glass.
 5. A method as described in claim 4 wherein the container is made of a borosilicate glass.
 6. A method as described in claim 4 wherein the container is made of stainless steel.
 7. A method as described in claim 5 further comprising the step of pre-heating the hard body and the container after evacuating and sealing the hard body in the container so that the borosilicate glass melts around the hard body.
 8. A method as described in claim 1 wherein the step of placing the hard body in the container further comprises wrapping the hard body in platinum foil prior to placing the hard body in the container.
 9. A method as described in claim 1 wherein the container is evacuated to at least about 20 mtorr.
 10. A method as described in claim 1 wherein the reaction mixture consists of particles having an average diameter of about 2 microns to about 10 microns.
 11. A method as described in claim 7 wherein the hard body and the borosilicate glass container are pre-heated to a temperature from about 700° C. to about 800° C. for a time from about 20 minutes to about 40 minutes at a pressure of at least about 100 psi after sealing the hard body in the container, prior to heating the hard body.
 12. A method as described in claim 6 wherein the Tl, Sr, Ca, and Cu oxides are reacted by heating the hard body and the container at a temperature from about 800° C. to about 950° C. and a pressure of at least 30,000 psi for a time from about 50 minutes to about 80 minutes.
 13. A method as described in claim 11 wherein the Tl, Sr, Ca, and Cu oxides are reacted by heating the hard body and the container at a temperature from about 850° C. to about 950° C. and a pressure of at least 30,000 psi for a time from about 25 minutes to about 40 minutes.
 14. A method of forming a Tl-Sr-Ca-Cu-oxide high T_(c) superconductor comprising the steps of:forming a powdered mixture of SrCO₃, CuO, CaCO₃ ; converting the carbonates in the powdered mixture to oxides, forming an oxide mixture; forming a reaction mixture of the oxide mixture and Tl₂ O₃ in stoichiometric proportions to make a Tl-Sr-Ca-Cu-oxide high T_(c) superconducting compound; compressing the reaction mixture into a hard body; placing the hard body into a container; evacuating and sealing the hard body in the container; heating the hard body and the container under pressure until said container melts around said hard body and the oxides of Tl, Sr, Ca, and Cu react to form a superconducting compound; and cooling the superconducting compound to room temperature and returning the superconducting compound to atmospheric pressure.
 15. A method as described in claim 14 wherein the Tl-Sr-Ca-Cu-oxide high T_(c) superconducting compound is selected from the group comprising TlSr₂ CaCu₂ O_(x) and TlSr₂ Ca₂ Cu₃₋₄ O_(x), wherein x is an integer from about 7 to about
 17. 16. A method as described in claim 14 wherein the carbonates are converted to oxides by calcining the powdered mixture in an air or oxygen atmosphere at from about 800° C. to about 1000° C.
 17. A method as described in claim 3 wherein the reaction takes place in a hot isostatic press.
 18. A method as described in claim 1 wherein the superconducting compound is cooled at such a rate so that the superconducting compound reaches room temperature and atmospheric pressure in from about 15 minutes to about 30 minutes. 